Our major objective has been, and will continue to be, the purification of methionine-t-RNA synthetase from mammalian liver. Our best preparation to date showed a 280-fold purification factor. This is almost double the purification reported by others. These workers postulated that met-t-RNA synthetase was a constituent of a complex of five aminoacyl t-RNA synthetases, since the ratio of activities remained constant over a 150-fold purification. Our data on relative ratios of activity of four of these enzymes (those for ile, leu, lys, and met) do not support this conclusion. We have recently obtained two fractions each of which exhibits met-t-RNA synthetase activity by stepwise elution from hydroxyapatite columns. The possibility that these fractions will show specificity towards t-RNA met (F) and t-RNA met (M) is being investigated. We have assayed a number of human tumors of the colon for met-t-RNA synthetase. The specific activity of the enzyme from the tumor is about double that from normal colon. Assay of this enzyme may be useful in diagnosis of tumors of the digestive tract.